1. Field of the Present Invention
The present invention relates to an apparatus for conducting a compression encoding on an image signal and particularly to an image signal compression encoding apparatus for achieving different processing for the respective signal components constituting image data.
2. Description of the Prior Art
When loading a memory with digital image data such as image data produced by an electronic still camera, in order to minimize the amount of storage data so as to reduce the storage capacity of the memory, various kinds of compression encoding have been employed. Particularly, the two-dimensional orthogonal transformation encoding is capable of accomplishing the encoding with a high compression ratio. Furthermore, a distortion of an image associated with the encoding can also be suppressed. Consequently, this encoding has been broadly adopted.
In the two-dimensional orthogonal transformation encoding, a field of image data is subdivided into a predetermined number of blocks to conduct the two-dimensional orthogonal transformation encoding on image data of each block thus produced. The resultant data, namely, a transformation coefficient is compared with a predetermined threshold value to truncate or round off a portion not exceeding the threshold value (coefficient truncation). Namely, the transformation coefficient not exceeding the threshold value is processed as data of zero in the subsequent processing. The obtained coefficient is then divided by a predetermined value of a quantization step, namely, a normalization coefficient. In other words, a quantization or normalization is achieved by use of the step width. This can suppress a value of the transformation coefficient, namely, a dynamic range of amplitude.
Subsequently, the normalized coefficient is encoded, for example, by use of the Huffman encoding, thereby completing the compression encoding.
In a case where the shooting of an image through an imaging device comprising photosensitive cells results in image data being subjected to the compression encoding, and the number of effective photosensitive cells varies as 200 thousand, 400 thousand, etc. among the commercially available imaging devices, the number of pixels forming the image data cannot be fixedly determined. Therefore, an image is subdivided in the compression encoding into blocks, the number of which is dependent upon the numbers of photosensitive cells included in the imaging device used. In consequence, when different encoding operations are necessary for direct-current, dc, and alternate-current, ac, components, respectively, a problem arises in which a switching between these operations cannot be accomplished depending on a preset criterion, which may, for example, be when a count of the blocks reaches a predetermined value.
For example, when the image data includes color image data, for each component of a luminance signal Y and color difference signals R-Y and B-Y, it is desirable to employ a different normalization and a different encoding operation. Namely, for example, since the luminance signal Y and color difference signals R-Y and B-Y possess frequency components different from each other, the normalization is to be desirably achieved with different normalization coefficients for the respective signals. Furthermore, since the transformation coefficient of each component comprises dc and ac components and the amount of data to be processed varies therebetween, the encoding thereof is required to be conducted depending on a Huffman table suitable for data of each component.
Consequently, when a compression encoding is carried out on such color image data, the normalization and the encoding are switched between the dc and ac components for each component of the luminance signal Y and color difference signals R-Y and B-Y. This necessitates the block count of image data of each component as the switching ciriterion to be a fixed value. In consequence, as described above, a disadvantage takes place in which for image data from solid-state imaging devices having different effective photosensitive cells, the compression encoding cannot be appropriately conducted.